If the 12" wrench isn't giving you enough leverage, get a bigger wrench or as suggesed, a pipe to fit on the end of you current wrench. Freewheels can be very tight. I have an 18" breaker bar that usually gets that duty.
If you are having trouble holding onto the wheel while applying all of that leverage, enlist a helper to steady the wheel for you. Mounting a tire on the wheel will also help as your helper will only need to push down on the wheel and let the tire's grip do the counterholding while you wrench on the freewheel.
If none of that works, find a vice and someone bigger than you to apply to force.
Since no one else has commented, counterclockwise is the correct direction to turn the freewheel.